1. Field of the Invention
The present invention generally relates to semiconductor memory devices, and particularly relates to a reduction of power consumption in a semiconductor memory device in a standby condition.
2. Description of the Prior Art
In order to shrink a size of transistors to step up an integration density of semiconductor memory devices such as DRAM, SRAM, flash memories, EPROM, etc., a concentration of impurities doped in the transistors needs to be increased. An increase in the impurity concentration narrows a width of depletion layers in the transistors, resulting in a larger electric field applied to the depletion layer. As a result, an inter-band tunnel current is generated, which is especially prominent in PMOS transistors. The inter-band tunnel current is caused by a large degree of bending of the band at the depletion layer, and flows even when a high voltage is applied to the gate of a PMOS transistor to turn it off.
FIG. 1 is an illustrative drawing for explaining an effect of the inter-band tunnel current. FIG. 1 shows an inverter circuit which is often used in word drivers for driving word-selection lines in a semiconductor memory device. The inverter circuit of FIG. 1 includes a PMOS transistor 201 and an NMOS transistor 202. In word drivers or the like in a semiconductor memory device, a high voltage (V.sub.DD) is applied to a gate G1 of the PMOS transistor 201 and a gate G2 of the NMOS transistor 202 when the semiconductor memory device is in a standby condition waiting for data input/output. This high voltage turns the PMOS transistor 201 off and turns the NMOS transistor 202 on, thereby turning an output of the inverter circuit into a low voltage.
With an intensified electric field applied to the depletion layer because of an increased integration density of the semiconductor memory device, however, an inter-band tunnel current I.sub.BT is generated in the PMOS transistor 201 of FIG. 1. The inter-band tunnel current I.sub.BT flows from a bulk B to a drain D in the PMOS transistor 201. As long as it remains a small amount of current, the inter-band tunnel current does not cause any detrimental effect to circuit operations.
In a semiconductor memory device, the inverter circuits as shown in FIG. 1 are used at various points across the chip. Word drivers for driving word-selection lines, in particular, include the inverter circuits of FIG. 1 for each of the word lines to which a high voltage is applied, so that the number of the inverter circuits is large, and the inter-band tunnel current flows in a large amount in each of these inverter circuits. Because of this, the inter-band tunnel current I.sub.BT in all the inverter circuits of the word drivers becomes significant in its total amount when the semiconductor device is in the standby condition.
Even when the semiconductor memory device is in operation for reading data, the inverter circuits of the word drivers are in a condition for outputting a low voltage level in unselected memory blocks. The inter-band tunnel current I.sub.BT in these inverter circuits are also significant in its total amount.
Power consumption caused by the inter-band tunnel currents is not preferable, especially when the semiconductor memory device is mounted in portable equipment, because it adds to the consumption of a battery. Portable equipment usually need a semiconductor memory device with low power consumption in order to make an efficient use of limited battery power.
Accordingly, the inter-band tunnel current should be reduced in a semiconductor memory device in order to suppress an amount of consumed current during the standby-condition period. It is known that the inter-band tunnel current is prominent generally in a turned-off transistor with a high voltage applied between a bulk and either a source or a drain. A reduction in the voltage between the bulk and either the source or the drain should thus result in a suppression of the inter-band tunnel current. Alternately, a change in the gate voltage to bring the transistor closer to a turned-on condition should decrease the inter-band tunnel current.
Accordingly, there is a need for a semiconductor memory device which can suppress an inter-band tunnel current in word drivers for driving word-selection lines so as to reduce a consumed current during the standby-condition period.